Abstract
A study of the freshwater mollusc diversity was conducted at selected sites in the relatively pristine Marico River and the impacted Crocodile River, the major tributaries of the Limpopo River. Four surveys were conducted, two in an early (May 2013 and 2014) and two in a late (November 2013 and 2014) low-flow period. Semi-quantitative surveys were done by sampling the vegetation, as well as the substratum, with a standard SASS net for approximately 15 min each. Environmental parameters including water temperature, electrical conductivity (EC) and pH were measured at each site. Molluscs were identified up to species level, sorted, counted, and the presence of juvenile specimens recorded. Historical data for the 1/16 th degree square grids (loci), in which each of the sampling sites of the current study was located, were extracted from the National Freshwater Snail Collection at the Unit for Environmental Sciences and Management, Potchefstroom Campus of the North-West University. During this study, 20 and 9 species were recovered from the Marico and Crocodile Rivers, respectively, as compared to 13 and 12 species on record for these loci, respectively. Juvenile specimens were present during the four surveys at most of the sites. Canonical correspondence analyses were applied which revealed that biotopes, water temperature and EC played the most significant role in the distribution and abundance of species. The relatively high mollusc diversity and the fact that juveniles were present throughout the study, demonstrated that current habitat and environmental conditions were suitable to promote recruitment and the sustainability of diverse mollusc populations in the Marico River and its tributaries. However, in contrast to this, the exploitation of and habitat transformation in the Crocodile River has resulted in the decrease of biotopes which eventually could have led to the decrease in diversity and the establishment of P. acuta , an exotic invader species. Keywords : freshwater molluscs, biodiversity, Marico River, Crocodile River, environmental variables
Highlights
Freshwater molluscs are the second most diverse animal phylum on earth (Lydeard et al, 2004)
From historical data in the database of the National Freshwater Snail Collection (NFSC) it is clear that the Marico and Crocodile River areas had a similar molluscan diversity, with only one more species recorded from the Marico River than the Crocodile (Table 4)
Of the 13 species on record for the Marico River area, only B. tropicus was not recovered during the present study, probably due to the fact that this snail is primarily found in lentic waterbodies such as dams and lakes and various ephemeral habitats, which were included in the previous surveys (De Kock et al, 2002)
Summary
Freshwater molluscs are the second most diverse animal phylum on earth (Lydeard et al, 2004). Little to no marginal and aquatic vegetation, pool with muddy substratum stones in current, riffle and run Marico River directly below Marico-Bosveld Dam
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